A suspension of fine coal particles in an aqueous medium, hereafter called coal/water slurry, is a relatively new fuel proposed for various combustion purposes. In particular, this fuel has been used in the past in boiler applications (as presented by Marnell and Krishna, 9th Energy Technology Conference and Expo., Washington, DC, Feb. 16-18, 1982) to generate heat that can be used directly or indirectly for the purpose of providing steam energy demands. This steam is subsequently used in industrial boilers to provide heat or in utility boilers to generate eletricity. In these studies, Marnell and Krishna concluded that this fuel was successfully burned and represented a viable alternative to burning other fuels, especially liquid fuels, in boilers.
The use of coal/water slurries in newly built combustion facilities is now being considered much more strongly than in the past due to the reduced availability of petroleum fuels and natural gas. Moreover, the use of these slurries is being very actively considered as a replacement for petroleum based liquid fuels in boilers. One of the key problems to be solved is how to make a coal/water slurry flame behave like a flame generated by a liquid fuel. Although this problem can be handled through design in newly built boilers, it presents a major obstacle in using slurries in existing boilers designed for petroleum liquids; that is, boilers retrofitted for coal/water slurries.
One particular variable, and perhaps the most important that affects boiler operations is ignition time. Recent studies by West, R. J., et al., AFRC-IFRF Symposium on Conversion to Solid Fuels, Paper 82-9: 1-51 (1982) indicate ignition of pulverized coal is difficult during start up of boilers. Coal/water slurries would make the ignition process even more difficult during start-up. Furthermore, due to the longer ignition times for coal/water slurries, the flame can become unstable during combustion at steady state and low load levels and "lock out". Once the flame goes out, an expensive time-consuming process of restarting the boiler must be carried out. Reports of stability problems with coal/water slurries, especially for low volatility coals, are noted in literature, such as J. A. Barsin, 9th Energy Technology Conference and Expo., Washington, DC, February 1982.
The earlier and the predominant number of coal/water slurry combustion tests required either or all of the following criteria: a preheated (to 1650.degree. F.) furnace as described by E. T. McHale, et al., Combustion and Flame, 45, 121 (1982) and G. Farthing, Jr., et al. EPRI Report CS-2286 (1982); a continuous pilot flame of natural gas (providing ca. 10-15% of the thermal input) as described by J. Barsin, Proceedings of Coal: Phoenix of the '80s, Halifax, Nova Scotia (1981) and Y. S. Pan, et al., Proceedings of the 4th International Symposium on Coal/Slurry Combustion, Vol. II, Session III (1982); or excessive, by industrial standards, combustion air preheat (400.degree.-500.degree. F.) as noted by the various authors just mentioned. As the state-of-the-art increased, the need for these three kinds of thermal support has reduced, but air preheating continues to remain a necessity, although at a level somewhere around 250.degree. F. as noted by R. K. Manfred, et al., ACS Division of Fuel Chemistry Reprints, 28.2, 36 (1983 ). This has been the case even though rather readily ignitable, high-volatile bituminous coals were loaded into the coal/water slurry.
Once ignited, coal-water slurries can only be burned over a relatively narrow range of operating conditions. For the most part, coal-water slurries can only be burned with excess air to achieve relatively high carbon conversion efficiencies (90+%). Sub-stoichiometric, or fuel rich, efficient combustion has not been successfully accomplished. This fact would deny to coal/water slurry burning the advantages of staged combustion for NO.sub.x control.
Because of the relatively poor ignition and stability characteristics of coal/water slurry flames, burner turndown ratios (rate of firing) have been rather low in the range of between 1:1 to 2:1. For slurries to replace petroleum liquids, the optimum turndown ratio would have to be approximately 4:1. Ignition and stability can be good and the turndown ratio still be poor as noted by G. Farthing, Jr. in the reference just cited above. Recently, some progress has been made toward improving burner turndown ratios as noted by R. K. Manfred, et al. (cited above), but still apparently only at high levels of excess air.
Another major variable that affects boiler efficiency during the use of coal/water slurries is carbon conversion efficiency. Carbon conversion is a measure of the degree to which the combustion of a coal particle is complete. When the carbon conversion efficiency is 100%, no carbon ends up with the ash as a by-product. The efficiencies of carbon conversion in pulverized coal-fired boilers are generally in the range of 95+% and are boiler and coal specific. Older or poorly-operated boilers and low volatile, high-ash coals have lower carbon conversion efficiencies, and the differences between these percentages and 100% represents costly and wasted fuel. Carbon conversion efficiencies for coal/water slurries are typically lower than for pulverized coal and range from approximately 92% to 95%.
Other problems experienced in boiler operations when using non-conventional particulate fuels such as wood, refuse, wastewater sludges, etc. either in a relatively dry form or in slurries containing water and/or oil and combinations thereof can also be alleviated using oxygen enriched air. That is, the benefits described herein for coal/water slurries in ignition time and carbon conversion improvements can also be gained when using these alternate non-conventional solid fuels, or slurries made therefrom.
Very little direct patent literature is available regarding coal/water slurry combustion with or without oxygen enrichment. However, several related patent references are noted. U.S. Pat. Nos. 4,394,137 and 4,326,856 discuss the generation and purification of synthesis gas produced by the partial oxidation of coal. In these cases, a coal/water mixture is fed to a reaction zone along with air enriched with oxygen to produce a high BTU synthesis gas composed primarily of CO and H.sub.2. The conditions required to maximize conversion to synthesis gas is very high pressure and lower temperatures when compared to combustion. In addition, oxygen enrichment is used primarily to reduce the nitrogen content of the product gas to maximize its BTU value, not to effect reaction kinetics per se. The reaction mechanisms, conditions, and rationale for using oxygen are totally different in these POX operations compared to coal combustion for boiler operations.
U.S. Pat. No. 4,211,174 indicates that coal can be oxidized in a slurry medium such as water to recover heat and sulfur-containing ash by-product. It was disclosed that oxygen enriched air can be employed to better achieve the desired oxidation results. In this process, the temperatures are very low compared to combustion (.about.400.degree.-700.degree. F. versus 3000.degree.-4000.degree. F.) and the reaction of oxygen in the coal takes place in a gas/liquid reactor system, at high residence times, with a catalyst to generate an approximate equilibrium composition of oxidizable and oxidized components. The conditions of temperature, pressure, and excessively long residence times makes this process very different from direct coal combustion.
U.S. Pat. No. 3,941,552 describes a process in which a coal/water/oil mixture is burned in a furnace. This reference is cited to indicate that many variations of pulverized solids and/or slurry media are being considered for commercial combustion applications, which could benefit from oxygen enrichment.